Sentaurus Structure Editor
6. Three-dimensional Structures

6.1 Overview
6.2 Reinitializing Sentaurus Structure Editor
6.3 Auto Region-Naming Mode
6.4 Selecting Materials
6.5 Selecting Default Boolean Expression
6.6 Creating a Cuboid Region
6.7 Changing the View and Zooming
6.8 Poly Reoxidation
6.9 Nitride Spacers
6.10 Defining Contacts
6.11 Setting Contacts at Existing Faces
6.12 Setting Contacts at New Faces
6.13 Defining Constant Doping Levels in a Region
6.14 Defining Analytic Doping Profiles
6.15 Defining a Meshing Strategy in a Window
6.16 Defining a Multibox Mesh Strategy in a Refinement Window
6.17 Saving the Model
6.18 Meshing the Device Structure
6.19 Assignment

Objectives

• To create three-dimensional structures using Sentaurus Structure Editor.

6.1 Overview

In Section 2. Generating 2D Boundaries, the steps for building a 2D SOI MOSFET were demonstrated. Sentaurus Structure Editor can also construct 3D structures. This section introduces the procedure to build a 3D MOSFET as shown in Figure 1.

Figure 1. Three-dimensional MOSFET. (Click image for full-size view.)

Due to the similarity of 2D and 3D operations, many commands and operations executed in this section are identical to those described in the 2D case. Therefore, cross references to relevant sections will be given to avoid duplication of text.

6.2 Reinitializing Sentaurus Structure Editor

See Section 2.2 Reinitializing Sentaurus Structure Editor.

6.3 Auto Region-Naming Mode

By default, Sentaurus Structure Editor automatically assigns region names such as region_1 and region_2 to newly created regions. This is useful in some applications but, in most cases, you may prefer to use your own names, which are more descriptive and easier to remember.

To switch off the automatic region-naming mode:

• Draw > Auto Region Naming and clear this mode.

When the mode has been switched off, you will be prompted to enter the region name whenever a new region is created. The corresponding Scheme command is:

(sdegeo:set-auto-region-naming OFF)

6.4 Selecting Materials

See Section 2.4 Selecting Materials.

6.5 Selecting Default Boolean Expression

See Section 2.5 Selecting the Default Boolean Expression.

6.6 Creating a Cuboid Region

To create a cuboid, silicon substrate region:

1. Switch off the auto region-naming mode.
2. Select Exact Coordinates.
3. Draw > Overlap Behavior > New Replaces Old or click the toolbar button () to select the default Boolean expression for resolving overlapping.
4. Select the Isometric View ().
5. Draw > Create 3D Region > Cuboid.
6. Select Silicon from the Material list.
7. Draw the cuboid in the view window:
   
   • Click to place the first corner of the cuboid and drag the pointer to the location of the opposite corner of the upper face (or lower face depending on the final placement of the cuboid) of the cuboid.
   • Finally, move the pointer (without pressing any button) to extend the face in the third direction. Click once to define the cuboid.
8. When the drawing is completed, the Cuboid Definition dialog box is displayed.
   Enter (-0.25 -0.2 0), (0.25 0.2 -1.0) to define the two opposite corners of the cuboid and click OK.
   
   

   Figure 2. Cuboid Definition dialog box.

9. Since the Auto Region Naming mode is switched off, a dialog box is displayed for the name of the region. Enter SubsSilicon.
10. Click OK.
    
    

    Figure 3. Dialog box to enter name of region.

11. If the cuboid does not fit the view, View > Zoom to Extents.

Other regions of the device, that is, the isolation trenches, the gate oxide, and the polysilicon gate can be created in a similar way. Use the following coordinates.

Region	Name	Material	Coordinates
Substrate	SubsSilicon	Silicon	(-0.25 -0.2 0), (0.25 0.2 -1.0)
Right trench	TrenchOxide_Right	SiO2	(-0.2 -0.2 0), (0.2 -0.1 -0.2)
Left trench	TrenchOxide_Left	SiO2	(-0.2 0.1 0), (0.2 0.2 -0.2)
Gate oxide	GateOxide	SiO2	(-0.15 -0.2 0), (0.15 0.2 0.002)
Poly gate	PolyGate	PolySi	(-0.1 -0.1 0.002), (0.1 0.2 0.1)

The corresponding Scheme commands for creating these regions are:

(sdegeo:create-cuboid (position -0.25 -0.2 0) (position 0.25 0.2 -1.0)  
 "Silicon" "SubsSilicon")
 
(sdegeo:create-cuboid (position -0.15 -0.1 0) (position 0.15 0.1 0.002)  
 "Oxide" "GateOxide")

(sdegeo:create-cuboid (position -0.2 -0.2 0) (position 0.2 -0.1 -0.2)  
 "Oxide" "TrenchOxide_Right")

(sdegeo:create-cuboid (position -0.2  0.1 0) (position 0.2  0.2 -0.2)  
 "Oxide" "TrenchOxide_Left")

(sdegeo:create-cuboid (position -0.1 -0.1 0.002) (position 0.1 0.2 0.1)  
 "PolySi" "PolyGate")

6.7 Changing the View and Zooming

Sentaurus Structure Editor supports functions that allow the constructed structure in the view window to be viewed from different perspectives.

• To view the structure from any of the axis directions, select one of these toolbar buttons:
  
  
  
  
• To rotate the structure, click the Orbit button (). Drag one point of the structure to rotate it in any direction.
• To zoom in or out, click the Zoom button. To zoom in, drag the pointer upwards to the left. To zoom out, drag the pointer downwards to the right.

Figure 4 and Figure 5 show the current status of the structure viewed from different angles.

Figure 4. Current status of 3D MOSFET structure. (Click image for full-size view.)

Figure 5. Current status of 3D MOSFET after using "orbit" tool to change the angle. (Click image for full-size view.)

6.8 Poly Reoxidation

To create a poly reoxidation region:

1. Change the default Boolean expression to "Old Replaces New" ().
   This ensures that the old regions of the structure are kept intact while new regions are defined and placed.
2. Create a cuboid that overlaps with the polysilicon gate region and another on top of the substrate surface. Follow the steps from Section 6.6 Creating a Cuboid Region and use the following parameter values:
   
   

   Region	Name	Material	Coordinates
   Poly reoxidation 1	PolyReOxide1	SiO2	(-0.103 -0.103 0) (0.103 0.2 0.1)
   Poly reoxidation 2	PolyReOxide2	SiO2	(-0.15 -0.103 0) (0.15 0.2 0.005)

The corresponding Scheme commands are:

(sdegeo:set-default-boolean "BAB")
(sdegeo:create-cuboid (position -0.103 -0.103 0) (position 0.103 0.2 0.1)  
 "Oxide" "PolyReOxide1")

(sdegeo:create-cuboid (position -0.15 -0.15 0) (position 0.15 0.2 0.005)
  "Oxide" "PolyReOxide2")

6.9 Nitride Spacers

To add nitride spacers:

1. Click the Old Replaces New button.
2. Create a cuboid using the following parameter values.
   
   

   Region	Name	Material	Coordinates
   Nitride spacers	NiSpacer	Si3N4	(-0.15 -0.15 0), (0.15 0.2 0.08)

To round the spacer corners and edges:

1. From the Selection Level list, select Select Vertex.
2. Click the Select button on the toolbar ().
3. Select the corner vertices of the spacer. Multiple vertices can be selected simultaneously by holding the Ctrl key when selecting.
   
   

   Figure 6. Selecting corner vertices of the spacer. (Click image for full-size view.)

4. Edit > Edit 3D > Fillet.
   A dialog box is displayed for input of the fillet radius.
5. Enter 0.03, and click OK.

Figure 7. Rounded corners of the spacer after using the parameter "fillet-edges". (Click image for full-size view.)

The corresponding Scheme commands are:

(sdegeo:create-cuboid (position -0.15 -0.15 0) (position 0.15 0.2 0.08)  
 "Si3N4" "NiSpacer")

(sde:define-parameter "fillet-radius" 0.03 0.0 0.0 )

(sdegeo:fillet-edges 
 (list (car (find-vertex-id (position -0.15 -0.15 0.08))) 
       (car (find-vertex-id (position  0.15 -0.15 0.08))) ) fillet-radius)

6.10 Defining Contacts

To define a contact:

1. Contacts > Contact Sets.
   The Contact Sets dialog box is displayed.
   
   

   

   Figure 8. Contact Sets dialog box. (Click image for full-size view.)

2. Type gate in the Contact Name field.
3. Click Set.
4. Other contacts, for example, source, drain, and substrate, can be defined in the same manner.
   Select a unique Face Pattern for each contact to allow for easy recognition later.
5. Click Close when all contacts have been defined.

The corresponding Scheme commands are:

(sdegeo:define-contact-set "gate"      4.0  (color:rgb 1.0 0.0 0.0 ) "##" )
(sdegeo:define-contact-set "drain"     4.0  (color:rgb 1.0 0.0 0.0 ) "||" )
(sdegeo:define-contact-set "source"    4.0  (color:rgb 1.0 0.0 0.0 ) "==" )
(sdegeo:define-contact-set "substrate" 4.0  (color:rgb 1.0 0.0 0.0 ) "<><>" )

6.11 Setting Contacts at Existing Faces

As in the 2D case, contacts do not become effective unless they are associated with part of the device structure. In 3D applications, contacts are typically defined on the faces of the device.

To place a substrate contact:

1. Select substrate from the Contact list.
   
   
   
   
2. Select Select Face from the Selection Level list.
3. Click the Select button ().
4. Select the bottom face of the silicon substrate.
5. Contacts > Set Faces to associate the substrate with the bottom of the substrate.
   (The bottom face is now decorated with the pattern set previously for the substrate contact.)
6. Repeat the previous steps and define the gate contact on top of the polysilicon surface.
   The source and drain contacts are, however, slightly different and require new faces to be created before they can be defined, which will be explained in the next section.

The corresponding Scheme commands are:

(sdegeo:set-current-contact-set "substrate")
(sdegeo:set-contact-faces (find-face-id (position 0 0 -1) ) "substrate")  

(sdegeo:set-current-contact-set "gate")

6.12 Setting Contacts at New Faces

The source and drain only occupy part of an existing face (the top silicon substrate face). Therefore, new faces must be created before these two contacts can be placed.

A convenient way to create a new face is, first, to create a metal cuboid to imprint the face:

1. Create a cuboid metal region for the source and one for the drain as discussed in Section 6.6 Creating a Cuboid Region. Use the following parameter values.
   
   

   Region	Name	Material	Coordinates
   Source cuboid	Source	Metal	(0.17 -0.1 0) (0.25 0.1 0.05)
   Drain cuboid	Drain	Metal	(-0.25 -0.1 0) (-0.17 0.1 0.05)

   
   
2. Remember these two regions are created only for the purpose of generating new faces on the device and are not a real part of the device structure. To remove them:
   
   • Select Select Body from the Selection Level list.
   • Click the Select button.
   • Select the two metal regions. To allow multiple regions to be selected at one time, hold the Ctrl key when selecting. All the selected regions are highlighted.
   • Edit > Remove > Region.
3. Although the two newly created metal regions have been removed immediately, the new faces created by the intersection of the two metal regions with the silicon top surface remain. The association of the source and drain contacts with these new faces can now be established using the technique from Section 6.11 Setting Contacts at Existing Faces.

The corresponding Scheme commands for creating and deleting the two metal regions are:

  "Metal" "Source")
(sdegeo:create-cuboid (position  0.17  -0.1 0) (position  0.25 0.1 0.05)
  "Metal" "Drain")
(sdegeo:delete-region (find-body-id (position -0.22 0 0.025)))
(sdegeo:delete-region (find-body-id (position  0.22 0 0.025)))

The corresponding Scheme commands for placing the source and drain contacts are:

(sdegeo:set-current-contact-set "source")
(sdegeo:set-contact-faces (find-face-id (position -0.22 0 0)) "source")  

(sdegeo:set-current-contact-set "drain")
(sdegeo:set-contact-faces (find-face-id (position  0.22 0 0)) "drain")  

6.13 Defining Constant Doping Levels in a Region

Defining a constant doping level in a 3D region is similar to that for a 2D region (see Section 3.3 Defining Analytic Doping Profiles).

To define a constant doping level of 1 x 10¹⁷ cm^-3 in the silicon substrate:

1. Device > Constant Profile Placement.
   The Constant Profile Placement dialog box is displayed.
   
   

   Figure 9. Constant Profile Placement dialog box.

2. Enter PlaceCD.Bulk in the Placement Name field.
3. In the Placement Type group box, select Region as the type, and select SubsSilicon from the list.
4. In the Constant Profile Definition group box, enter Const.Bulk in the Name field.
5. Select BoronActiveConcentration from the Species list.
6. Enter 1e17 in the Concentration field.
7. Click Add Placement.
8. Repeat these steps to define an ArsenicActiveConcentration of 10²⁰ cm^-3 in the polysilicon gate region.
9. Click Close.

The corresponding Scheme commands are:

(sdedr:define-constant-profile "Const.Bulk" "BoronActiveConcentration" 1e17)
(sdedr:define-constant-profile-region "PlaceCD.Bulk"
  "Const.Bulk" "SubsSilicon")

(sdedr:define-constant-profile "Const.Poly" "ArsenicActiveConcentration" 1e20)
(sdedr:define-constant-profile-region "PlaceCD.Poly"
  "Const.Poly" "PolyGate")

6.14 Defining Analytic Doping Profiles

Defining an analytic doping profile in a 3D region is comparable to the 2D case. The only difference is that, in the 3D case, the reference for the profile is a 2D plane. In the 2D case, it is a 1D edge.

As in the 2D case, the definition of a profile in a 3D region involves two steps. The first step defines the baseline window and the second defines the profile itself. The baseline window determines the lateral extent of the profile and serves as the reference point for the depth of the peak position of the profile.

The following instructions define the source and drain implants, both in Gaussian shape, with a peak concentration of 10¹⁹ cm^-3, a junction depth of 0.1 μm, and a lateral straggle/diffusion factor of 0.8.

To define a baseline window:

1. Draw > Exact Coordinates.
2. Mesh > Define Ref/Eval Window > Rectangle.
3. In the view window, draw a rectangle.
4. In the Exact Coordinates dialog box, enter (0.30 -0.25) and (0.15 0.25) to define the rectangle.
5. Click OK.
6. In the dialog box that is displayed, enter BaseLine.Source for the baseline name.
7. Repeat these steps for the drain baseline definition. Use the following parameter values.
   
   

   Junction	Baseline name	Start point	End point
   Source	BaseLine.Source	(0.30 -0.25)	(0.15 0.25)
   Drain	BaseLine.Drain	(-0.15 -0.25)	(-0.30 0.25)

The corresponding Scheme commands for these two definitions are:

(sdedr:define-refinement-window "BaseLine.Source" "Rectangle"  
 (position  0.30 -0.25 0.0)  (position 0.15 0.25 0.0) )

(sdedr:define-refinement-window "BaseLine.Drain" "Rectangle" 
 (position -0.30 -0.25 0.0)  (position -0.15 0.25  0.0) )

To define and place the Gaussian doping profiles:

1. Device > Analytic Profile Placement.
   The Analytical Profile Placement dialog box is displayed.
   
   

   Figure 10. Analytical Profile Placement dialog box.

2. Enter PlaceAP.Source in the Placement Name field.
3. Select BaseLine.Source from the Ref/Win list.
4. In the Profile Definition group box, enter Gauss.SourceDrain in the Name field.
5. Select ArsenicActiveConcentration from the Species list.
6. Enter 1e19 in the Peak Concentration field.
7. Enter 0 in the Peak Position field.
8. Enter 1e17 in the Junction field and 0.1 in the Depth field.
9. Enter 0.8 in the Lateral Diffusion Factor field.
10. Click Add Placement.
11. Repeat these steps and define the drain implant.
    Note that the profile definition Gauss.SourceDrain can be reused but the baseline needs to change to BaseLine.Drain.
12. Click Close.

The corresponding Scheme commands are:

(sdedr:define-gaussian-profile "Gauss.SourceDrain"
  "ArsenicActiveConcentration" "PeakPos" 0.0 "PeakVal" 1e19
  "ValueAtDepth" 1e17 "Depth" 0.1 "Gauss" "Factor" 0.8)

(sdedr:define-analytical-profile-placement "PlaceAP.Source"
 "Gauss.SourceDrain" "BaseLine.Source" "Both" "NoReplace" "Eval")

(sdedr:define-analytical-profile-placement "PlaceAP.Drain"
 "Gauss.SourceDrain" "BaseLine.Drain" "Both" "NoReplace" "Eval")

6.15 Defining a Meshing Strategy in a Window

To generate a meshing strategy that is globally effective, first create a refinement window covering the entire device area, then define a meshing strategy, and apply it to the window.

To generate a meshing strategy:

1. Activate the Exact Coordinates mode.
2. Mesh > Define Ref/Eval Window > Cuboid.
   A dialog box is displayed for the name of the refinement box.
3. Enter RefWin.Global.
4. Click OK.
5. Another dialog box is displayed for the location of the refinement window.
   Enter (-0.25 -0.2 -0.5), (0.25 0.2 0.1) in the corresponding fields.
6. Click OK.
7. Mesh > Refinement Placement.
   The Refinement Specification dialog box is displayed.
   
   

   Figure 11. Refinement Specification dialog box. (Click image for full-size view.)

8. Enter Place.Global as the Placement Name.
9. Select Ref/Eval Window as the Placement Type and RefWin.Global as the refinement window.
10. In the Refinement Definition group box, enter RefDef.Global in the Name field.
11. Enter the Max Element Size and Min Element Size for all directions: Xmax = Ymax = Zmax = 0.1 and Xmin = Ymin = Zmin = 0.05.
12. Click Add Placement.
13. Repeat these steps and define a tighter refinement strategy in the top active region of the device. Use RefWin.Active as the refinement box name and the coordinates (-0.25 -0.2 -0.15) (0.25 0.2 0.0) for the refinement window. Use Place.Active as the placement name and set Xmax = Ymax = Zmax = 0.025 and Xmin = Ymin = Zmin = 0.0125.
14. Click Close.

The corresponding Scheme commands are:

(sdedr:define-refinement-window "RefWin.Global" "Cuboid"
  (position -0.25 -0.2 -1.0) (position 0.25 0.2 0.1))

(sdedr:define-refinement-size "RefDef.Global"
  0.1  0.1  0.1
  0.05 0.05 0.05 )

(sdedr:define-refinement-placement "Place.Global"
 "RefDef.Global" "RefWin.Global" )

(sdedr:define-refinement-window "RefWin.Active" "Cuboid"
 (position -0.25 -0.2 -0.15) (position 0.25 0.2 0.0))

(sdedr:define-refinement-size "RefDef.Active"
  0.025  0.025  0.025
  0.0125 0.0125 0.0125 )

(sdedr:define-refinement-placement "Place.Active"
"RefDef.Active" "RefWin.Active")

6.16 Defining a Multibox Mesh Strategy in a Refinement Window

To add a multibox refinement strategy in the channel region:

1. Mesh > Multibox Placement.
   The Multibox Specification dialog box is displayed.
   
   

   Figure 12. Multibox Specification dialog box. (Click image for full-size view.)

2. Enter PlaceMB.Channel in the Placement Name field.
3. Select the option Define Ref/Eval Window, enter the coordinates (-0.12 -0.1 -0.05) and (0.12 0.1 0.0) for the cuboid window definition, and click Define. Enter the window name RefWin.Channel in the dialog box that is displayed, and click OK.
4. In the Placement Type group box, select RefWin.Channel from the list.
5. In the Multibox Definition group box, enter MB.Channel in the Name field.
6. Enter the values for the maximum and minimum grid spacing for the directions:
   Xmax = Ymax = 0.1 and Zmax = 0.01, Xmin = Ymin = 0.05 and Zmin = 0.001.
7. For the X and Y ratios, enter 0.0. For the Z ratio, enter -1.5.
8. Click Add Placement.
9. Click Close.

Figure 13 shows the device after these steps with refinement windows.

Figure 13. Three-dimensional MOSFET with refinement windows. (Click image for full-size view.)

The corresponding Scheme commands for placing the multibox are:

(sdedr:define-refinement-window "RefWin.Channel"
 "Cuboid" (position -0.12 -0.1 -0.05) (position 0.12 0.1 0))
 
(sdedr:define-multibox-size "RefDefMB.Channel"
 0.1  0.1  0.01
 0.05 0.05 0.001
 1    1   -1.5 )

(sdedr:define-multibox-placement "PlaceMB.Channel"
  "RefDefMB.Channel" "RefWin.Channel" )

6.17 Saving the Model

To save the model for later use:

• File > Save Model, or Ctrl+S, or click the corresponding toolbar button.

The corresponding Scheme command is:

(sde:save-model "3dmos_sde")

where 3dmos_sde is the saved file name. See also Section 2.14 Saving the Model for the various file types saved.

6.18 Meshing the Device Structure

To create the mesh and save the device structure:

1. Mesh > Build Mesh.
   The Build Mesh dialog box is displayed.
   
   

   Figure 14. Build Mesh dialog box. (Click image for full-size view.)

2. Enter n1_MOSFET3D in the Save Grid to File field.
3. Select SNMESH as the meshing engine.
4. Select Tecplot SV as the mesh viewer.
5. Click Build Mesh.

The structure along with its doping conditions are saved in a TDR file n1_MOSFET3D_msh.tdr, which can be loaded into, for example, Sentaurus Device simulations.

Figure 15 shows the device after the meshing step.

Figure 15. Three-dimensional MOSFET with its doping and meshing conditions. (Click image for full-size view.)

The corresponding Scheme command is:

(sde:build-mesh "snmesh" "" "n1_MOSFET3D")

Click to view all the commands discussed in this section in the command file MOSFET3d_dvs.cmd.

The complete project can be investigated from within Sentaurus Workbench in the directory Applications_Library/GettingStarted/sde/MOSFET3D.

6.19 Assignment

Create a FinFET structure similar to the one shown in Figure 16.

Figure 16. Three-dimensional FinFET structure with its doping and meshing conditions.

Click to view a solution of the command file FinFET_dvs.cmd.

The complete project can be investigated from within Sentaurus Workbench in the directory Applications_Library/GettingStarted/sde/FinFET.